There are many large deposits of oil shale in the midwestern United States, notably in Colorado and other adjacent states. Oil shale may include calcium magnesium carbonite or dolomite, and organic kerogen material. At elevated temperatures, the decomposition of the organic kerogen contained in the oil shale produces combustible product gases, and shale oil from which fuel and other useful products may be made. Incidentally, the amount of shale oil which may be obtained from oil shale varies considerably with the amount of kerogen included in the oil shale, and may range from a few gallons of shale oil which may be obtained per ton of oil shale, up to as much as 60 or 70 gallons per ton in very rich oil shale. In oil shale formations the richer layers or beds of high grade oil shale will frequently extend through a large formation, often very nearly horizontally, or at a slight angle to the horizontal.
There are two principal broad methods for obtaining the combustible gaseous products and the shale oil from oil shale. The first of these methods involves above-ground retorting, where the rich layers of oil shale are mined from the geological formation and are transported to retorts, normally located near the mining area, for above-ground retorting, in accordance with known techniques. Within the oil shale formation, the mining would characteristically be conducted using both room and pillar mining techniques, in a rich layer of oil shale. In this regard, unless the oil shale was moderately rich in kerogen content, the amount of shale oil and combustible gases which could be obtained would not be sufficiently great to justify the expense of mining and hauling the oil shale to the surface, and the capital investment in the above-ground retorting equipment.
Now, returning to the mine itself, when using so-called "room and pillar" techniques, the height of the rich layer of oil shale which would normally be mined would be about 50 to 70 feet. The supporting pillars which would be left would be characteristically about 50 to 65 feet square, and the spaces between the pillars about 45 feet to 60 feet. Large horizontal areas, perhaps 3,000 to 3,500 feet by perhaps 800 to 950 feet would be blocked off by extended panel barrier pillars. These barrier pillars are required to provide stable long-term haulage and access areas during the mining operation, as well as to isolate sections of the mine for ventilation purposes.
The other principal retorting technique which has been proposed involves what has been termed "modified in-situ" oil shale retorting. Typical patents which disclose in-situ retorting arrangements include U.S. Pat. Nos. 4,133,580 granted Jan. 9, 1979, inventor Gordon B. French; U.S. Pat. No. 4,153,298 granted May 8, 1979, inventors: Harry E. McCarthy and Gordon B. French; and U.S. Pat. No. 4,230,367, granted Oct. 28, 1980, inventor Harry E. McCarthy. In general, in-situ retorted involves the formation of a rubblized mass of oil shale which is characteristically of substantially greater vertical height than horizontal extent. The retorting volume is preferably filled with moderately uniformly sized pieces of oil shale, and the total void volume included within the in-situ retort volume is normally about 15% to 30% of the total volume of the retort. The retorts are normally formed by mining out several horizontal sections from the substantial vertical extent of the intended retort location, and then blasting both from above and below into the mined out volumes. Suitable input and output conduits are located at the top and the bottom of the retort for supplying air and for removing product gases, and suitable drainage arrangements are provided at the bottom of the retort for collecting shale oil, and water, in cases where steam is injected into the retort along with air.
Now, considering the merits of the two different types of retorting systems or techniques as described above, the above-ground retorting arrangements permit a high level of control, and are effective in extracting a large proportion of the shale oil and product gases which are available from the mined oil shale. However, above-ground retorts are capital intensive, and it is relatively expensive to mine the oil shale and transport it to the retorting location. On the other hand, in-situ retorting requires very little capital equipment, as the oil shale formation itself forms the retort. However, when the entire process takes place within the formation, control both as to the sizing of the oil shale and also as to the progress of the retorting is difficult, and accordingly, the yield both in terms of output shale oil and product gases may be somewhat less than for the above-ground retorting method.
Concerning other factors which are involved, in the conventional room and pillar mining of oil shale for above-ground retorts, a fairly large fraction of the high grade shale is left behind in the pillars (typically, 50% to 60%), and a very large fraction of the total oil shale in the formation above and below the high grade zone selected for room and pillar mining is left behind unmined as not being sufficiently rich for the highly capital invensive surface retorts. It is a possibility that some of the oil shale operators might mine a second or even additional horizontal layers of the higher grade shale at some later date, but this is uncertain, and the pillars and the remainder of the shale would still not be utilized. The total resource recovery from such an operation might normally be in the range of 10 to 18% of the total oil shale deposit.
Accordingly, a principal object of the present invention is to provide a secondary oil shale recovery technique, for use following conventional room and pillar operations for above-ground retorting.